Representatives of subclass Hirudinomorpha have often been neglected in numerous freshwater ecosystem research. The results of conducted investigations including leeches indicate their importance as bioindicators of water quality and changes in the environment. Leeches have the ability to accumulate toxic compounds in their tissue (e.g. PCB), thus indicating negative anthropological influence on water habitats. Prior to their application in ecological research, they have been, and are still used, in the treatment of different human health problems.